tests: Fix KeyFile test so it works from harness

tests: Fix KeyFile test so it works from harness

The auth/keys test tries to find a file that's distributed as part
of the test suite. However, it currently only looks in the CWD to
find it. Modify the test so that if it's run from the test harness,
it will use the harnesses SOURCE environment variable to locate the
KeyFile

Change-Id: I93e16a01eae79b38ab01c81a57d2a47c28479b27
Reviewed-on: http://gerrit.openafs.org/4213
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>